Process and plant for the heterogeneous synthesis of chemical compounds

ABSTRACT

A process for the heterogeneous synthesis of chemical compounds such as methanol and ammonia through catalytic conversion of the respective gaseous reactants that are made to pass through a first ( 2 ) and a second ( 3 ) reaction zone connected in series with each other, in which they react in pseudoisothermal conditions, distinguishes itself in that in the first reaction zone ( 2 ) the gaseous reactants are made to flow through a fixed mass of an appropriate catalyst in which a plurality of substantially box-like, plate-shaped heat exchangers ( 21 ), arranged side-by-side and crossed by a heat exchange operating fluid, is dipped.

FIELD OF APPLICATION

The present invention relates, in its broader aspect, to a process forthe heterogeneous synthesis of chemical compounds such as methanol andammonia.

In particular, the present invention relates to a process of the typecomprising two reaction zones connected in series with each other inorder to carry out catalyzed chemical reactions in so calledpseudoisothermal conditions, wherein the reaction temperature iscontrolled within a restricted range of values around a predeterminedoptimal value.

The present invention also relates to a plant for carrying out theaforesaid process.

PRIOR ART

In the field of the industrial production of chemical compounds such asmethanol and ammonia, the need is well known of developing processes ofheterogeneous synthesis with a high conversion yield of the reactantsand plants with large capacities, at low investment costs and energyconsumption.

In order to fulfil the aforesaid need, a process for methanol synthesishas been proposed in the art, comprising two reaction zones connected inseries with each other and operating in pseudoisothermal conditions,i.e. with reaction heat removal, wherein the heat in excess formed inthe second reaction zone is removed by indirect heat exchange with theflow of fresh and recycled reactants fed into the first reaction zone.

Such process is described in EP-A-0 790 226. In order to operatecorrectly and reach the desired economical advantages, it is howevernecessary that the first reaction zone be consisting of a tube bundleexchanger, with the corresponding tubes filled with a suitable catalyst.The tubes are internally crossed by the gaseous reactants H₂ and CO,whereas externally they are licked by a water flow (with steamproduction) as heat exchange operating fluid. A reactor of this type isfor example described in U.S. Pat. No. 4,559,207.

The need to employ this specific kind of reactor in the first reactionzone of a two-step process for the synthesis of methanol is alsoconfirmed in GB-A-2 203 427.

Although advantageous under various aspects, the above described processhas a relevant and acknowledged technical drawback, which constitutes,at an industrial level, a sure limit for the advancement or completiondegree of the chemical reaction considered (conversion yield) as well asof the productive capacity of the respective plant.

In fact, the tube bundle reactors just described imply a complexity ofstructure and use such as to only allow the manufacture of rather smallreaction volumes as clearly indicated in EP-A-0 790 226, with thedisadvantage of impairing the conversion yield and the productivecapacity that can be obtained by such reactors.

For larger reaction volumes the tube bundle reactors, besides being ofvery difficult if not impossible application, require such a high amountof investments that the process with a two-step reaction is no longercost-effective.

In order to overcome such drawback, GE-A-2 203 427 proposes to use ahigh efficiency catalyst, which, besides solving only partially theproblem of the low conversion and production yield in the tube bundlereactors, is however very expensive.

As a result, because of the aforesaid disadvantages, the processesaccording to the prior art do not allow to obtain in a relativelycost-effective and technically simple and reliable way, high conversionyields and high production capacities.

SUMMARY OF THE INVENTION

The technical problem underlying the present invention is that ofproviding a process for the heterogeneous synthesis of chemicalcompounds such as methanol and ammonia, which is easy to develop andallows high conversion yields to be obtained in chemical plants withlarge capacities at low investment costs and energy consumption,overcoming the drawbacks of the prior art.

The above indicated technical problem is solved, according to theinvention, by a process for the heterogeneous synthesis of chemicalcompounds such as methanol and ammonia through catalytic conversion ofthe respective gaseous reactants that are made to cross through a firstand a second reaction zone connected in series with each other in whichthey react in pseudoisothermal conditions, which process ischaracterized by the fact that in said first reaction zone the gaseousreactants are made to flow through a fixed mass of an appropriatecatalyst in which a plurality of substantially box-like, plate-shapedheat exchangers, arranged side-by-side and crossed by the heat exchangeoperating fluid, is dipped.

Advantageously, contrary to the constant teaching of the prior art, ithas been surprisingly found that the conversion yield and the productioncapacity of the first reaction zone in a process of the above describedtype can be remarkably increased, in a simple, reliable andcost-effective way, thanks to the aforesaid features.

In doing so, it is possible to produce the aforesaid chemical compoundsin large amounts and with a high conversion yield in large capacitychemical plants, which are technically simple to be developed and do notimply high energy consumption and high investment and maintenance costs.

The invention further relates to a chemical plant having structural andfunctional features suitable to carry out the aforesaid process.

The features and advantages of the process according to the inventionwill be clearer from the description of an indicative and not-limitingembodiment thereof, made with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows in a general and schematic way a block diagram illustratinga plant for carrying out the process according to an embodiment of thepresent invention;

FIG. 2 schematically shows in longitudinal section a detail of the plantrepresented by the block diagram of FIG. 1.

DETAILED DESCRIPTION OF THE DRAWINGS

In FIG. 1 there is schematically illustrated in all of its maincomponents a plant for methanol or ammonia production according to thepresent invention, which is indicated in its whole with numeral 1.

Plant 1 comprises a first reaction zone 2 and a second reaction zone 3,connected in series with each other.

Inside the reaction zones 2 and 3, a reaction area 4 is provided tohouse, in a per se known way, a fixed mass of a suitable catalyst, notshown.

The reaction zones 2 and 3, when in function, do operate inpseudoisothermal conditions and therefore are equipped with heatexchange units 5 and 6, respectively, dipped into said catalyst in thereaction area 4.

The reaction temperature inside the area 4 of the first reaction zone 2is controlled through indirect heat exchange by making a heat exchangefluid to flow inside the unit 5, as indicated by the arrows. A heatexchange fluid such as for example water in case of exothermalreactions, such as methanol or ammonia synthesis. During such crossing,the water is transformed into steam or is simply preheated for thefollowing production of steam in dedicated boilers placed outside thereaction zone and not shown.

The reaction temperature inside the area 4 of the second reaction zone 3is instead controlled by indirect heat exchange, by making a flow ofgaseous reactants, for feeding into the first reaction zone 2, to flowinside the heat exchange unit 6 as heat exchange fluid. In this respect,a pipe 7, in fluid communication with the heat exchange unit 6, entersinto the second reaction zone 3 at such unit 6, then comes out of thesame and enters into the first reaction zone 2 in the reaction area 4.

The pipe 7, as well as the unit 6, is crossed by a flow of gaseousreactants, such as H₂ and CO for methanol synthesis and H₂ and N₂ forammonia synthesis, both fresh and recycled.

Furthermore, a pipe indicated with numeral 8 puts in fluid communicationthe outlet of the area 4 of the first reaction zone 2 with the inlet ofthe area 4 of the second reaction zone, for feeding thereto a reactionmixture comprising methanol or ammonia and unreacted gaseous reactantsobtained in the first reaction zone 2.

Exiting from the area 4 of the second reaction zone 3, a pipe 9 isfinally arranged for the outlet of the end reaction mixture, comprisingalso a portion of unreacted gaseous reactants beside methanol orammonia.

In a section of the plant of FIG. 1 in fluid communication with the pipe9 and not shown as it is per se conventional, the methanol and ammoniaso obtained are separated from the reaction mixture and the gaseousreactants present in such mixture are recycled into the first reactionzone 2 through the pipe 7 together with the fresh feed gaseousreactants.

According to a feature of the present invention, the heat exchange unit5, besides being dipped into the catalyst of the reaction area 4, ismade up of a plurality of substantially box-like, plate-shaped heatexchangers, arranged side-by-side and crossed by the heat exchangeoperating fluid, as can be seen in FIG. 2 which represents in greaterdetail the first reaction zone 2.

In such figure, the first reaction zone 2 consists of a pseudoisothermalreactor comprising a cylindrical shell 10, closed at the opposite endsby respective upper 11 and lower 12 bottoms and enclosing a heatexchange unit 5 provided with plate-shaped elements, which will beillustrated in the following description.

The upper bottom 12 is provided with a nozzle 13 for the inlet into thereactor 2 of the gaseous reactants coming from the pipe 7 of FIG. 1, andwith nozzles 14, 15 for the inlet and outlet of the heat exchangeoperating fluid in and from the heat exchange unit 5, respectively.

The lower bottom 11 is instead equipped with a nozzle 16 for the outflowfrom the reactor 2 of the reaction mixture in fluid communication withthe pipe 8 of FIG. 1.

Inside the shell 10 the reaction area 4 is provided, which comprises anannular catalytic bed 17, known per se, open above and with theside-walls perforated, for a radial or axial-radial crossing of the bedby the gaseous reactants.

The inner side-wall of the catalytic bed 17 forms in its interior apassage 18, closed above by a cover 19 and in fluid communicationthrough a joint 20 with the nozzle 16 for the outlet of the reactionmixture.

In the reaction area 4, and more precisely inside the catalytic bed 17,the heat exchange unit 5 is supported, in a per se conventional way, tobe dipped in the mass of an appropriate catalyst, not represented.

According to this embodiment, the heat exchange unit 5 has asubstantially cylindrical configuration and comprises a plurality offlattened, substantially box-like, plate-shaped heat exchangers 21 witha parallelepiped configuration, placed side-by-side in an arrangementwith coaxial and concentric elements (substantially radial arrangement).

More in particular, although not represented, each heat exchanger 21 ispreferably made up of a pair of juxtaposed metallic plates mutuallyjoined in a predetermined distanced relationship by perimetricsoldering, so that a chamber 21 a (illustrated with a dotted line) isdefined between them, intended for being crossed by the heat exchangeoperating fluid.

Each heat exchanger 21 is provided, at its opposite long sides 22, witha distribution pipe 23 and a collector pipe 24, respectively, for saidoperating fluid. The pipes 23 and 24 are in fluid communication, on oneside, with said chamber 21 a through at least one, but preferablythrough a plurality of openings or holes (not represented), of whichthey are provided with along one or more generatrices and, on the otherside, with the space outside the exchanger 21 through respective inletand outlet tubular joints 25 and 26, for said operating fluid. Thejoints 25 and 26 are in turn connected with the nozzles 14 and 15,respectively.

In order to facilitate the crossing by the heat exchange operating fluidof the heat exchanger 6 in radial or substantially radial direction, thechamber 21 a is preferably divided into a plurality of partitions, notdirectly in communication with each other and obtained, for example,through a corresponding plurality of welding seams or separating baffles(indicated with a dotted line) extending perpendicularly to thedistributing pipe 23 and to the collector pipe 24 of the exchanger 21.

Thanks to this embodiment of the first reaction zone 2, it is possibleto carry out the process according to the present invention, in whichthe gaseous reactants are made to flow through a fixed mass of asuitable catalyst of such reaction area, in which a plurality ofsubstantially box-like, plate-shaped heat exchangers, arrangedside-by-side and crossed by the heat exchange operating fluid, isdipped.

In doing so, it is advantageously possible to develop in a simple,reliable, and economic way and with low energy consumption even largespaces (volumes) of reaction for the first reaction zone 2.

In other words, the presence of plate-shaped heat exchangers dipped intothe catalytic mass, besides being particularly effective as indirectheat exchange elements, allow the sizing of the first reaction zone 2 tobe carried out at will, and thus to obtain in such reaction zone a highconversion yield and a high production capacity, to the advantage of theglobal conversion yield as well as to the development of plants with alarge capacity.

The invention thus conceived may be susceptible to variations andmodifications, all falling within the scope of protection defined in thefollowing claims.

For example, according to a preferred embodiment of the presentinvention, the reaction mixture coming from the first reaction zone 2and fed to the second reaction zone 3 through the pipe 8, isadvantageously cooled by means of indirect heat exchange in a heatexchanger 27—of the conventional type—illustrated with a dotted line inFIG. 1. In this way, not only is it possible to recover heat in order toproduce, for example, steam to be used in other parts of the steam plantbut, above all, it is possible to control the inlet temperature to thesecond reaction zone 3 and hence its conversion yield.

Alternatively, it is also possible to foresee that a part of the “fresh”gaseous reactants and/or of the recycled reactants be directly fed tothe first reaction zone 2, through a pipe 28, without passing throughthe second reaction zone 3.

The heat exchange unit 6 may be of a conventional type, i.e. of the tubebundle type or else in the form of a serpentine pipe, or, advantageouslyit can also be made up of a plurality of plate-shaped heat exchangers ofthe type described with reference to FIG. 2. In doing so, it is possibleto obtain a further increase of the conversion yield and of theproduction capacity of the chemical plant.

According to a further embodiment of the invention, not represented, thefirst and the second reaction zone 2, 3 may be enclosed in a singlesynthesis reactor, instead of having two reactors as in the example ofFIG. 1.

The operating conditions of temperature inside the reaction zones arethe conventional ones for methanol or ammonia synthesis. As far as thepressure operating conditions are concerned, particularly satisfyingresults have been obtained by operating the two reaction zones 2 and 3substantially at the same pressure, and preferably between 50 and 100bars for the methanol synthesis and between 50 and 300 bars, preferablybetween 80 and 150 bars, for ammonia synthesis.

1. A process for the heterogeneous synthesis of methanol or ammoniathrough catalytic conversion of the respective gaseous reactantscomprising the steps of passing the gaseous reactants through a firstand a second reaction zone connected in series with each other, in whichthe gaseous reactants react in pseudoisothermal conditions, and causing,in said first reaction zone, the gaseous reactants to flow through afixed mass of an appropriate catalyst in which a plurality ofsubstantially box-like, plate-shaped heat exchangers arrangedside-by-side and crossed by a heat exchange operating fluid, is dipped.2. The process according to claim 1, wherein the pressure inside saidreaction zones is the same.
 3. The process according to claim 1, whereinsaid gaseous reactants are fed into said first reaction zone afterindirect heat exchange inside said second reaction zone with a reactionmixture fed into this latter reaction zone and coming from said firstreaction zone.
 4. The process according to claim 1, wherein said secondreaction zone is fed with a reaction mixture coming from said firstreaction zone and subjected beforehand to indirect heat exchange inorder to control its inlet temperature into said second zone.
 5. Theprocess according to claim 1, wherein said first reaction zone is fedwith a mixture of gaseous reactants comprising fresh gaseous reactantsand recycled gaseous reactants, the latter being suitably separated by areaction mixture coming from said second reaction zone.